1. Field of the Invention
The present invention relates to an image searching apparatus, a control method thereof, a program, and a storage medium, in particular to a technology that searches for an image that is similar to a queried image from among a plurality of comparative target images that include color images and grayscale images.
2. Description of the Related Art
A variety of image searching apparatuses have been put forth over time that employ color information or luminance information as an image feature parameter. A search for a similar image using color information, i.e., a color information search, however, merely performs a registration process using extracted color characteristics, and performs a search according to a comparison that takes color into consideration. A search for a similar image using luminance information, i.e., a luminance information search, on the other hand, merely performs a registration process using extracted luminance characteristics, and performs a search according to a comparison that takes luminance into consideration. The color information search and the luminance information search are thus treated as distinct processes, and the search results are accordingly outputted distinctly from one another. Consequently, it has heretofore not been possible to perform a unified search that leverages the strengths of the respective search technologies.
A configuration does exist, however, that resolves such a lack of utility by facilitating a search when comparative target images are color or grayscale images alike, by extracting the luminance information when the search source image is a grayscale image, even if the comparative target images are color images, per Japanese Patent laid-open No. 2004-334335.
The pre-existing configuration, however, did not perform a comparison of grayscale images when the search source image is a color image, even if grayscale images were present, instead treating only the color images as the comparative target images. Consequently, it has heretofore not been possible to perform a unified search using a single search technology.
When the comparative target images are grayscale images, the configuration that Japanese Patent laid-open No. 2004-334335 discloses treats the grayscale images as color images, and extracts and stores the color features, which are actually the grayscale features. Treating the grayscale images as color images involves assigning the same value to each of the RGB color channels, thus precluding obtaining features relating to color qualities, thereby dramatically lowering the degree of similarity between the grayscale images and the color images. Consequently, it is not possible to accurately perform a search for similar images using the configuration that Japanese Patent laid-open No. 2004-334335 discloses.